A typical throttle body assembly in present-day use takes the form of a one-piece metal casting formed with a main bore or flow passage extending through the body along a first axis which is intersected by a throttle shaft bore extending through the body along a second axis perpendicular and intersecting the axis of the main bore. A circular throttle plate is mounted within the main bore on a shaft rotatably supported in the shaft bore. By rotating the shaft, the plate can be moved between a blocking or minimum flow position, in which the plate is generally perpendicular to the axis of the main bore, to a maximum open position in which the plate is aligned with the bore axis to thereby meter flow through the main bore.
Throttle control valve systems have recently been developed which override the driver's command as represented by his positioning of the accelerator pedal by taking over control of the throttle. These systems are frequently referred to as “electronic throttle control” or “drive-by wire” systems in that there is no direct mechanical connection between the accelerator pedal and the throttle, the pedal position being transmitted to an electronic control unit as an electric signal which is processed by the control unit and transmitted by the control unit in the form of an electric signal to an electromechanical throttle actuator. This type of “drive-by wire” system may also be adapted for sensing the speed of the engine driven wheels with respect to that of the non-driven wheels, so that if the speed of the driven wheels becomes greater than that of the non-driven wheels, the control unit can transmit an electric signal to the electromechanical actuator which alters the position of the throttle plate to reduce the torque output of the engine until the engine driven wheels regain traction.
While such “drive-by wire” systems, in general, are quite reliable, a malfunction of the electrical supply system or the electronic control unit can result in a total loss of control by the driver over the throttle valve position in that the sole control over the position of the throttle valve is an electrical signal from the electronic control unit, and there is no direct mechanical connection between the accelerator pedal and the throttle valve. In the worst case, an electrical or electronic malfunction could result in unintended and uncontrollable acceleration of the vehicle, should the throttle plate, upon the loss of its electrical positioning signal, be in or moved to a wide open position. Consequently, most throttle plates are spring-biased to move to a closed position in response to loss of an electrical control signal, rendering the vehicle more or less effectively immobilized in that the idle air flow will be reduced to a minimum.